Cd-free Buffer Layers Research Articles

CuIn(Ga)Se2 Solar Cell Fabricated By All Wet-Processes

CuIn(Ga)Se2, I-III-IV chalcopyrite compound semiconductors, are attractive for thin film solar cells. Recently, energy conversion efficiency of CuIn(Ga)Se2 solar cells has been achieved over 20% based on vacuum process. To reduce fabrication cost, many efforts have been focused on non-vacuum process, and in this study we tried to fabricate the cells by almost all wet-processes; CuIn(Ga)Se2 absorption layers and In2S3 buffer layers were electrochemically deposited, and ZnO window layers were formed by solution based chemical methods. The deposited CuIn(Ga)Se2 absorption layers were heat-treated to improve crystallinity; however, due to surface cracks formed during heat-treatment, it was hard to adopt electrochemical method to form the absorption layers. Authors solved the problem to compensate the cracks by two-step electrodeposition process. Cd-based thin films had been used as buffer layers for a while, but nowadays many studies reported Cd-free buffer layers for CuIn(Ga)Se2 solar cells due to environmental destruction by toxicity of Cd. Therefore, we used In2S3 buffer layers forming by electrochemical method instead of Cd-bsed thin films. ZnO window layers were then deposited by solution based process in compact wire forms. We will discuss in this presentation about microstructures, chemical compositions, and light absorption abilities of each layer. And solar cell efficiency will also be presented.

Effect of Indium Doping on Surface Optoelectrical Properties of Cu2ZnSnS4 Photoabsorber and Interfacial/Photovoltaic Performance of Cadmium Free In2S3/Cu2ZnSnS4 Heterojunction Thin Film Solar Cell

Maximum conversion efficiency of 6.9% was obtained over an electrodeposited Cu2ZnSnS4-based thin film solar cell with a Cd-free In2S3 buffer layer by applying a rapid post-heat treatment to the In2S3/Cu2ZnSnS4 stacked layer. It was found that post-heating of the In2S3/Cu2ZnSnS4 stack promoted an increment of the acceptor density of the Cu2ZnSnS4 layer close to the In2S3–Cu2ZnSnS4 heterointerface of the In2S3/Cu2ZnSnS4 stack. Moreover, the diffusion of In also resulted in a red-shift of the band gap energy of Cu2ZnSnS4 from 1.47 to 1.40 eV. Due to extension of external quantum efficiency response of the solar cell to the long wavelength region, the solar cell based on the post-heated In2S3/Cu2ZnSnS4 stack reached appreciably large short circuit current density of more than 20 mA cm–2. The energy difference between the conduction band minimum of In2S3 and that of Cu2ZnSnS4 at the In2S3/Cu2ZnSnS4 heterointerface was determined to be a slightly positive value of 0.11 eV, indicating formation of a “notch-type”...

Amorphous Indium Selenide Thin Films Prepared by RF Sputtering: Thickness-Induced Characteristics.

The influence of indium composition, controlled by changing the film thickness, on the optical and electrical properties of amorphous indium selenide thin films was studied for the application of these materials as Cd-free buffer layers in CI(G)S solar cells. Indium selenide thin films were prepared using RF magnetron sputtering method. The indium composition of the amorphous indium selenide thin films was varied from 94.56 to 49.72 at% by increasing the film thickness from 30 to 70 nm. With a decrease in film thickness, the optical transmittance increased from 87.63% to 96.03% and Eg decreased from 3.048 to 2.875 eV. Carrier concentration and resistivity showed excellent values of ≥1015 cm(-3) and ≤ 10(4) Ω x cm, respectively. The conductivity type of the amorphous indium selenide thin films could be controlled by changing the film-thickness-induced amount of In. These results indicate the possibility of tuning the properties of amorphous indium selenide thin films by changing their composition for use as an alternate buffer layer material in CI(G)S solar cells.

Non-Stoichiometric Amorphous Indium Selenide Thin Films as a Buffer Layer for CIGS Solar Cells with Various Temperatures in Rapid Thermal Annealing.

The conventional structure of most of copper indium gallium diselenide (Culn(1-x)Ga(x)Se2, CIGS) solar cells includes a CdS thin film as a buffer layer. Cd-free buffer layers have attracted great interest for use in photovoltaic applications to avoid the use of hazardous and toxic materials. The RF magnetron sputtering method was used with an InSe2 compound target to prepare the indium selenide precursor. Rapid thermal annealing (RTA) was conducted in ambient N2 gas to control the concentration of volatile Se from the precursor with a change in temperature. The nature of the RTA-treated indium selenide thin films remained amorphous under annealing temperatures of ≤ 700 degrees C. The Se concentration of the RTA-treated specimens demonstrated an opposite trend to the annealing temperature. The optical transmittance and band gap energies were 75.33% and 2.451-3.085 eV, respectively, and thus were suitable for the buffer layer. As the annealing temperature increased, the resistivity decreased by an order-of-magnitude from 10(4) to 10(1) Ω-cm. At lower Se concentrations, the conductivity abruptly changed from p-type to n-type without crystallite formation in the amorphous phase, with the carrier concentration in the order of 10(17) cm(-3).

Room-Temperature Chemical Solution Treatment for Flexible ZnS(O,OH)/Cu(In,Ga)Se2 Solar Cell: Improvements in Interface Properties and Metastability.

We demonstrate an effective room-temperature chemical solution treatment, by using thioacetamide (S treatment) or thioacetamide-InCl3 (In-S treatment) solution, on Cu(In,Ga)Se2 (CIGSe) surface to engineer the ZnS(O,OH)/CIGSe interface and junction quality, leading to enhanced efficiency and minimized metastability of flexible solar cells. The control device without treatment reveals a relatively low efficiency of 8.15%, which is significantly improved to 9.74% by In-S treatment, and 10.39% by S treatment. Results of X-ray photoelectron spectroscopy suggest that S is incorporated into CIGSe surface forming CIGSSe by S treatment, whereas a thin In-S layer is formed on CIGSe surface by In-S treatment with reduced amount of S diffusing into CIGSe. PL spectra and TRPL lifetime further reveal that S incorporation into CIGS surface may substitute the OSe and/or directly occupy the vacant anion site (VSe), resulting in the effective passivation of the recombination centers at CIGSe surface. Moreover, reducing the concentrations of VSe may thereby decrease the density of (VCu-VSe) acceptors, which can minimize the metastability of ZnS(O,OH)/CIGSe solar cells. With S treatment, the light soaking (LS) time of ZnS(O,OH)/CIGSe device is reduced approximately to one-half of control one. Our approach can be potentially applied for alternative Cd-free buffer layers to achieve high efficiency and low metastability.

Solution-processed In2S3buffer layer for chalcopyrite thin film solar cells

We report a route to deposit In2 S3 thin films from air-stable, low-cost molecular precursor inks for Cd-free buffer layers in chalcopyrite-based thin film solar cells. Different precursor compositions and processing conditions were studied to define a reproducible and robust process. By adjusting the ink properties, this method can be applied in different printing and coating techniques. Here we report on two techniques, namely spin-coating and inkjet printing. Active area efficiencies of 12.8% and 12.2% have been achieved for In2 S3 -buffered solar cells respectively, matching the performance of CdS-buffered cells prepared with the same batch of absorbers.

ZnS buffer layer for Cu2ZnSn(SSe)4 monograin layer solar cell

Copper zinc tin sulfo-selenide Cu2ZnSn(SSe)4 (CZTSSe) is a low-cost alternative semiconductor material that can be used as an absorber in solar cells. CdS deposited by chemical bath deposition (CBD) is the most efficient buffer layer for Cu2ZnSn(SSe)4 and Cu(InGa)(SSe)2 (CIGS) solar cells. However, there is a strong demand for the development of a Cd-free buffer layer due to the toxicity of Cd and its associated concerns with respect to the disposal and long term safety of Cd containing solar modules. In this work, we report for the first time, the successful use of a ZnS buffer layer for CZTSSe monograin solar cell that shows similar functionality level as a CdS buffer layer. ZnS buffer layer was deposited onto CZTSSe absorber layer by employing a scalable non-vacuum CBD method. The effect of morphology, thickness, as well as chemical composition of the ZnS buffer layer on the efficiency of the CZTSSe solar cell was investigated, and the best CZTSSe monograin solar cell had an efficiency of 4.50 (±0.16)%. External quantum efficiency (EQE) showed higher transmission in the blue light region for the ZnS buffer compared to CdS. Increased number of ZnS layers decreased the EQE signal in 400–800nm regions, resulting in decreased J–V parameters, suggesting a single layer of 10–25nm ZnS as a most efficient alternative buffer for CZTSSe.

Junction and Back Contact Properties of Spray-Deposited M/SnS/In2S3/SnO2:F/Glass (M = Cu, Graphite) Devices: Considerations to Improve Photovoltaic Performance

SnS/In2S3 heterojunction devices were fabricated entirely by chemical spray pyrolysis in a superstrate configuration on SnO2:F/glass. The SnS/In2S3 junction was found to exhibit strong rectification behavior, and the Mott–Schottky characteristics showed it was abrupt. The photovoltaic behavior of the junction was investigated under air mass 1.5G illumination, showing a short-circuit current of 4.8 mA/cm2 and an open-circuit voltage of 0.29 V, reportedly the highest to date among similar devices with a Cd-free buffer layer and processed by a nonvacuum technique. However, the device suffers from low fill factor due to high series resistance originating from interface inhomogeneities. A Cu back contact was associated with a low level of inhomogeneities at the interface, as demonstrated by impedance analysis.

Adaptation of the surface-near Ga content in co-evaporated Cu(In,Ga)Se2 for CdS versus Zn(S,O)-based buffer layers

In this work, we show that in order to optimize the efficiency of Cu(In1-x,Gax)Se2 (CIGS) solar cells with Cd-free Zn(S,O)-based buffer layers, the Ga concentration in the CIGS absorber layer towards the hetero-interface has to be adapted. We varied the In and Ga deposition rates in the last stage of our 3-stage co-evaporation process, leading to different compositional ratios xf=[Ga]/([Ga]+[In]) between 0.15 and 0.6 in the top 400nm of the absorber layer. All absorber layers were then completed with both CdS and Zn(S,O) buffer layers by chemical bath deposition. While cells with our standard grading of xf≈0.4 in the front region result in a best performance of 15% with a CdS buffer, similar efficiencies with a Zn(S,O) buffer layer are only obtained when the Ga content near the hetero-interface is reduced down to x≈0.25. The maximum efficiency for the CdS buffer layer coincides with the maximum open circuit voltage (Voc) and fill factor (FF). Interestingly, for the Zn(S,O) buffer layer, this is not the case: the Voc increases steadily for higher Ga ratios, while the FF is fairly constant for 0.25<x <0.5 and decreases drastically for more extreme values. The findings are explained by differences in the conduction band offsets which result from the conduction band shift close to the surface due to Ga content variations. The results illustrate the importance of the absorber layer adaptation for different buffer layers and are an important step on the way to Cd-free buffer layers.

Growth of a High-quality Zn(S,O,OH) thin film via chemical bath deposition for Cd‐free Cu(In,Ga)Se2 solar cells

This study focused on the characterization and optimization of a Zn(S,O,OH) thin film via chemical bath deposition (CBD) on a Cu(In,Ga)Se2 (CIGS) film in order to obtain a reproducible and high-quality Cd-free buffer layer. High-resolution images of the actual film growth during the CBD process were observed to deeply understand the growth conditions for a uniform and pinhole-free Zn(S,O,OH) film on the CIGS film. The homogeneous precipitation of Zn(S,O,OH) was suppressed through the complexation reaction in the solution, and the heterogeneous precipitation of Zn(S,O,OH) was activated on the CIGS film by increasing the NH3 concentration. At NH3 concentration of 7M, the Zn(S,O,OH) film grown on the CIGS film completely covered the CIGS surface via the heterogeneous precipitation of Zn(S,O,OH). However, it contained many pinholes. The pinholes were eliminated by suppressing the formation of the Zn(OH)2 precipitates at a lower temperature (73°C). At the optimized conditions of the CBD process, the buffer film grown on the CIGS film contained a large amount of oxygen in the form of Zn–O and Zn–OH bonds. By annealing the Zn(S,O,OH) film, the content of the Zn–OH bond decreased through dehydration and that of the Zn–O bond increased. The JSC of the cell performance was greatly improved by annealing the Zn(S,O,OH) film and a conversion efficiency of 14.2% with VOC=0.62V, JSC=35.1mA/cm2, and FF= 65.1 was achieved.

Effect of Temperature on Structural and Optical Properties of Chemically Deposited Tin Sulfide Thin Films Suitable for Photovoltaic Structures

SnS (tin sulphide) is of interest for use as an absorber layer and the wider energy band gap phases e.g. SnS2, Sn2S3and Sn/S/O alloys of interest as Cd-free buffer layers for use in thin film solar cells. Thin films of tin sulphide have been deposited using CBD at three different bath temperatures (27, 35 and 45 °C) onto microscope glass substrates. The X ray diffraction (XRD) analysis of the deposited films reveled that all films has orthorhombic SnS phase as dominant one with preferred orientations along (111) direction. The temperature influence on the crystalline nature and the presence of other phases of SnS has been observed. The average grain size in the films determined from Scherers formula as well as from Williamson-Hall-plot method agrees well with each other. Energy dispersive X-ray (EDAX) analysis used to determine the film composition suggested that films are almost stoichiometric. The scanning electron microscopy (SEM) reveals that deposited films are pinhole free and consists of uniformly distributed spherical grains. The optical analysis in the 200-1200 nm range suggests that direct allowed transitions are dominant in the absorption process in the films with variation in the band gap (~1.79 to ~2.05 eV) due to variation in deposition temperature.

Inline Cu(In,Ga)Se$_{2}$ Co-evaporation for High-Efficiency Solar Cells and Modules

In this paper, co-evaporation of Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) in an inline single-stage process is used to fabricate solar cell devices with up to 18.6% conversion efficiency using a CdS buffer layer and 18.2% using a Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> Cd-free buffer layer. Furthermore, a 15.6-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> mini-module, with 16.8% conversion efficiency, has been made with the same layer structure as the CdS baseline cells, showing that the uniformity is excellent. The cell results have been externally verified. The CIGS process is described in detail, and material characterization methods show that the CIGS layer exhibits a linear grading in the [Ga]/([Ga]+[In]) ratio, with an average [Ga]/([Ga]+[In]) value of 0.45. Standard processes for CdS as well as Cd-free alternative buffer layers are evaluated, and descriptions of the baseline process for the preparation of all other steps in the Ångström Solar Center standard solar cell are given.

Cd-free CIGS solar cells with buffer layer based on the In2S3 derivatives

This study guided by device evaluations was conducted to reveal the reasons for the loss of the photo-generated carriers in CIGS cells with the buffer based on In2S3 derivatives. Chemical bath deposited Inx(OOH,S)y films have been employed as a Cd-free buffer layers. When compared to solar cells with CdS buffer layer, the Cu0.9(In0.7,Ga0.3)Se2.1 (Eg = 1.18 eV) cells with the Inx(OOH,S)y buffer exhibited strong voltage-dependent carrier collection and poor spectral response above 500 nm, presumably, due to energy barrier at the junction. In order to improve the charge collection by upward shift of the conduction band minimum of CIGS absorber, Inx(OOH,S)y/Cu0.9(In0.55,Ga0.45)Se2.1 (Eg = 1.30 eV) solar cells were also fabricated and their spectral responses were examined. When compared to the Cu0.9(In0.7,Ga0.3)Se2.1 cells, the improved spectral response and voltage dependent carrier collection were obtained. Nevertheless, considerable loss in charge collection above 500 nm was still observed. The efficiency reached 9.3% while the Cu0.9(In0.7,Ga0.3)Se2.1 cell exhibited only the efficiency of 3.4%. Finally, CIGS (Eg = 1.18 eV) solar cells with n-ZnO/i-ZnO/Inx(OOH,S)y/CdS/CIGS and n-ZnO/i-ZnO/CdS/Inx(OOH,S)y/CIGS configurations were fabricated. The influence of the TCO/buffer interface on the device characteristics was also addressed by means of comparison between the characteristics of two cells employing different interfaces. A 13.0% efficient cell has been achieved from n-ZnO/i-ZnO/CdS/Inx(OOH,S)y/CIGS configuration. The obtained data suggested that the limitation of the device efficiency was mainly related to the i-ZnO/Inx(OOH,S)y interface. The experimental results provide the knowledge base for further optimization of the interface properties to form high-quality p-n junction in the CIGS solar cells employing the CBD In2S3 buffer layer.

Effects of additives on the improved growth rate and morphology of Chemical Bath Deposited Zn(S,O,OH) buffer layer for Cu(In,Ga)Se2- based solar cells

ABSTRACTZn(S,O,OH) Chemical Bath Deposited (CBD) remains one of the most studied Cd-free buffer layer for replacing the CBD-CdS buffer layer in a Cu(In,Ga)Se2-based (CIGSe) solar cells and has already demonstrated its potential to lead to high-efficiencies. However, in order to further increase the deposition rate of the Zn(S,O,OH) layer during the CBD, the inclusion of additives can be a reasonable strategy, as long as the efficiencies of solar cells are maintained. The aim of this work is to understand the effect of the introduction of additives such as hydrogen peroxide (H2O2), H2O2+ethanolamine (C2H7NO) and H2O2+tri-sodium citrate (Na3C6H5O7) during CBD on the deposition mechanism, the growth rate and the quality of the buffer layer. It has been shown that the combined use of H2O2 and citrate in the bath formulation allows the deposition of Zn(S,O,OH) via a mix of “ion-by-ion” and “cluster-by-cluster” mechanisms that have good properties as buffer layers leading to high efficiency solar cells.

Flexible Cd-free Cu(In, Ga)Se2 solar cells with non-vacuum process

Non-vacuum processing combining with roll-to-roll manufacturing technology is available to fabricate large-area and low-cost flexible thin film solar cell. In this work, Cu(In, Ga)Se2 (CIGS) solar cells were fabricated on flexible stainless steel substrates with the CIGS absorber layer prepared using ink-printing technique and selenization process. A Cr diffusion barrier layer was sputtered onto the substrate before the deposition of Mo back contact to prevent the diffusion of detrimental elements like Fe from the substrate. The Cd-free buffer layer of In2S3, instead of the traditional CdS, was deposited on the surface of CIGS absorber layer for non-toxic and environmentally friendly consideration. The CIGS absorber layer exhibits a bilayer structure with the upper layer with large CuInSe2 grains and the bottom layer with small Cu(In, Ga)Se2 grains. The best performance of flexible CIGS solar cell achieves to an efficiency of 11.1%, a Voc of 0.45V, a Jsc of 37.7mA/cm2 and a fill factor of 0.66 with an active area of 0.14cm2.

Growth Behavior of Nanocrystalline ZnS Thin Films for Solar Cell Using CBD Technique

CdS buffer layers deposited by chemical bath deposition (CBD) have been used in high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells. In view of the harmful environmental impact of Cd due to high amount of Cd compound waste from large scale solar cell production, Cd-free buffer layers should be developed. In this work CBD method was used to deposit nanocrystalline zinc sulfide (ZnS) thin films for CIGS solar cell. The growth behavior containing deposit morphology, growth rate, structure and optical properties of ZnS thin films within 100nm thick is investigated by a field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), atomic force microscope (AFM) and ultraviolet visible light spectroscope (UV/VIS). The FE-SEM morphologies showed that the impact, even and dense ZnS films were deposited on soda lime glass from solution contained high concentration ratio of thiourea. The TEM diffraction patterns and XRD patterns revealed that ZnS films were nanocrystallines, of which FCC (111) phase dominated. The growth rate and transmittance of ZnS thin films are 0.88nm/min and over 80%, respectively. The ZnS/CIGS/FTO structure and Al:ZnO/ZnO/ZnS structure in CIGS solar cell formed by wet processes showed good heterointerfaces.

Chemical deposition methods for Cd-free buffer layers in CI(G)S solar cells: Role of window layers

It is currently possible to prepare Cd-free Cu(In,Ga)Se 2-based solar cells with efficiencies similar or higher than their CdS references. In these cells, higher efficiencies are generally obtained from soft chemical-based techniques giving conformal depositions such as chemical bath deposition (CBD), ion layer gas reaction (ILGAR) or atomic layer deposition (ALD). However most of these devices are characterized by their pronounced transient behaviour. The aim of this paper is to compare these different chemical-based methods (CBD, ALD, ILGAR…) and to try to provide evidence for the dominant influence of the interface between the Cd-free buffer layer and the window layer on the performance and on the metastable electronic behaviour of these solar cells.

Sputtered ZnO-based buffer layer for band offset control in Cu(In,Ga)Se 2 solar cells

Buffer layers for high-efficiency Cu(In,Ga)Se 2 (CIGS) solar cells are commonly prepared by chemical bath deposition. The highest efficiencies are obtained using a CdS buffer layer. In view of the detrimental environmental impact of Cd and to improve in-line processing for mass production, Cd-free buffer layers fabricated by a dry process are attractive. In this study, ZnO 1 − x S x thin films were prepared by co-sputtering of ZnO and ZnS targets and they were used as the buffer layer in CIGS solar cells. The conduction band offset (CBO) between ZnO 1 − x S x /CIGS layers was controlled by varying the sulfur content x in ZnO 1 − x S x . CIGS solar cells with an In 2O 3:Sn/ZnO 1 − x S x /CIGS/Mo/soda–lime glass structure were fabricated with different x. The variations in the solar cell parameters with the sulfur content were consistent with that of the CBO. The maximum efficiency was obtained at x = 0.18 and it was > 90% that of CdS/CIGS solar cells.

I-II-V half-Heusler compounds for optoelectronics:Ab initiocalculations

Half-Heusler compounds $XYZ$ crystallize in the space group $F\overline{4}3m$ and can be viewed as a zinc-blende-like ${(YZ)}^{\ensuremath{-}}$ lattice partially filled with He-like ${X}^{+}$ interstitials. In this work, we investigated I-II-V (eight-electrons) half-Heusler compounds by first-principles calculations in order to find suitable semiconductors for optoelectronics such as Cd-free buffer layer materials for chalcopyrite-based thin-film solar-cell devices. We report a systematic examination of band gaps and lattice parameters, depending on the electronegativities and the ion radii of the involved elements. Half-Heusler buffer materials should have a band gap of more than 2 eV to avoid absorption losses and a lattice constant of about $5.9\text{ }\text{\AA{}}$ to match the crystal structure of the absorber material. With these criteria we selected seven half-Heusler compounds as candidates for a buffer layer material.

In situ investigation of wet chemical processes for chalcopyrite solar cells by L-edge XAS under ambient conditions

Two instrumental setups for in situ soft X-ray absorption spectroscopy in liquid systems are demonstrated in this work. One for investigating chemical reactions in solutions and a new one for the solid component of a liquid / (as in both / absorber) solid interface. We used these setups for investigating two production processes for chalcopyrite solar cells under ambient conditions, probing the L-edge of Zn and Cu. The first one is a flow cell with a silicon nitride membrane to study the chemical bath deposition process for Cd-free buffer layers. Examining the electronic structure of involved Zn complexes allows to determine the exact reaction mechanism taking place during this process. The second setup is a rotating disk for investigating the bath/absorber interface upon the etching process of superficial binary copper compounds of the absorber as a function of time. The time resolution of the chemical reaction demonstrated in this study ranges from the second to minute time scale.